Conventional power-factor correction circuits generally employ a method in which current peaks of a switching element are made proportional to an input instantaneous voltage. According to this method, it is theoretically possible to obtain an input current that is proportional to the input instantaneous voltage.
FIG. 1 is a diagram showing an example of such a conventional power-factor correction circuit. In this power-factor correction circuit, it is detected that a current flowing in a reactor L2 has reached zero, when the voltage of an auxiliary winding Nd of the reactor L2 reaches or falls below a threshold Vth. When the current of the reactor L2 reaches zero, a flip-flop circuit FF1 is set to turn on a switching element Q1. In this way, the switching current of the switching element Q1 appears as a triangle-wave signal starting from zero.
Moreover, a switching element Q2 is turned off using an inverted signal from an inverter circuit INV1 at the same time as when the switching element Q1 is turned on. As a result, a capacitor C1 is charged with a constant current from a constant current source Ict, so that a voltage Vct of the capacitor C1 rises. A comparator CP1 compares the voltage of a capacitor C2 and the voltage Vct of the capacitor C1, and the output of the comparator CP1 shifts to a H level and resets the flip-flop circuit FF1 when the voltage Vct exceeds the voltage of the capacitor C2. As a result, the switching element Q1 is turned off. At the same time, the inverter circuit INV1 inverts a L-level signal which is the output of the flip-flop circuit FF1 and applies a H-level signal to the gate of the switching element Q2. As a result, the switching element Q2 is turned on, and the voltage Vct of the capacitor C1 is reset to zero.
As the above-described operation is repeated, the ON period of the switching element Q1 is controlled to be long when the output of an error amplifier A1 increases, and the ON period of the switching element Q1 is controlled to be short when the output of the error amplifier A1 decreases. The error amplifier A1 is configured to compare a reference voltage Vref and a value obtained by dividing the output voltage with a resistor R1 and a resistor R2, and operate in such a way that the output of the error amplifier A1 decreases as the output voltage rises, and the output of the error amplifier A1 increases as the output voltage drops.
As a result, the ON periods of the switching element Q1 are controlled such that the output voltage can be a target value which is determined by the reference voltage Vref and the ratio of voltage division by the resistor R1 and the resistor R2. During this control, the ON periods are set in such a way as to prevent response to the frequency of the AC input voltage, with the help of phase correction by the capacitor C2 provided at the output of the error amplifier A1. Accordingly, the ON periods of the switching element Q1 remain substantially the same during a half cycle of the AC input.
Since the inclination of the current flowing in the reactor L2 is proportional to the input instantaneous voltage, the switching element Q1 is controlled with substantially the same ON periods during the half cycle of the input AC voltage. Thus, as shown in FIG. 2, peaks Isw of a switching current Iin appear as triangle waves proportional to an input AC voltage instantaneous value Vin, and the average value thereof is proportional to the input AC voltage. In reality, however, stray capacitance is present in parallel with elements such as the switching element Q1, a diode D2, and the reactor L2.
For this reason, in a case where the peak current of the switching element Q1 is equal to or below a certain level, the energy stored in the reactor L2 while the switching element Q1 is turned off is all consumed to charge the stray capacitance. Consequently, the anode voltage of the diode D2 fails to exceed the output voltage, and no current flows to the output side. That no current flows to the output side means that no current flows in from the input. This leads to a phenomenon in which the input current does not flow when the input AC voltage is equal to or below a certain level (FIG. 3). Thus, there is a problem in that, in a range where the input instantaneous voltage is low, the input current does not flow, thereby resulting in a narrower conduction angle and a deteriorated power factor. Published Japanese Translation of PCT International Application No. 2006-526975 and Japanese Patent Application Publication No. 2008-199896 have been known as techniques that solve this problem.
In the circuit described in Published Japanese Translation of PCT International Application No. 2006-526975, a rectified voltage obtained by rectifying an input AC voltage is detected, and the ON period of a switching element is modified to be shorter than that determined by output voltage control when the input voltage instantaneous value is large, whereas the ON period is modified to be longer when the input voltage instantaneous value is small. In this way, when the input voltage instantaneous value is small, the switching current is increased and the time for which the input current does not flow is shortened. Accordingly, the conduction angle can be widened and the power factor can be improved.
Moreover, Japanese Patent Application Publication No. 2008-199896 utilizes a fact that a forward voltage generated in a secondary wiring of a reactor in a power-factor correction circuit when a switching element Q1 is turned on is proportional to an input instantaneous voltage. The ON period of the switching element is controlled to be long when the forward voltage of the secondary winding of the reactor is low, whereas the ON period is controlled to be short when the forward voltage is high.
In this way, in Japanese Patent Application Publication No. 2008-199896, like Published Japanese Translation of PCT International Application No. 2006-526975, in a range where the input instantaneous voltage is low, the ON period is increased, thereby providing a more current than the normal switching current which is proportional to the input instantaneous voltage. Accordingly, the conduction angle can be widened and the power factor can be improved.
However, in Published Japanese Translation of PCT International Application No. 2006-526975, a rectified voltage obtained by rectifying an alternating current is detected. The voltage of the power-factor correction circuit after the rectification is high because it is a voltage obtained by rectifying a commercial alternating current. Thus, in a case of inserting a voltage detection circuit, a circuit design and selection of components taking high voltage into consideration are necessary.
In the case of Japanese Patent Application Publication No. 2008-199896, the input voltage is not detected directly, and therefore the high-voltage problem does not occur. However, for a power-factor correction circuit for critical mode control, it is possible to employ a method in which the switching current is detected directly and the auxiliary winding of the reactor is not utilized. Then, to employ this method, a different auxiliary winding is necessary.
An object of the present invention is to provide a power-factor correction circuit capable of widening the conduction angle and improving the power factor, without detecting the input voltage or detecting the voltage of an auxiliary winding of a reactor.